Cytotoxic T-lymphocyte-associated protein 4 (CTLA4) is an immune checkpoint protein, expressed by T cells, which has a critical role in regulating the immune response. The finely-tuned nature of CTLA4 mediated immune regulation in humans is illustrated by the fact that heterozygous loss of function mutations in CTLA4 cause a syndrome of severe autoimmunity. We recently reported that another immune regulatory disorder, very similar in appearance to haploinsufficiency of CTLA4, but caused by biallelic mutations in ?LPS- responsive vesicle trafficking, beach and anchor containing? (LRBA), is associated with functional deficiency of CTLA4 and is highly responsive to therapy with CTLA4-Ig. Furthermore, we demonstrated that LRBA interacts with the cytoplasmic tail of CTLA4, altering its intracellular distribution, and slowing lysosomal degradation. Our biochemical experiments demonstrated interaction between the BEACH domain of LRBA and the YVKM motif of CTLA4, the same motif bound by clathrin adaptors. Thus, we hypothesize that LRBA controls CTLA4 levels and localization via a direct BEACH:YVKM interaction, in competition with clathrin adaptors, preventing movement to lysosomal compartments. We propose to test this hypothesis by first assessing structural and biophysical LRBA: CTLA4 interactions. Second, we will make an unbiased assessment of interacting proteins via mass spectrometry for both LRBA and CTLA4. In so doing we will define proteins which potentially modify or mediate LRBA:CTLA4 interactions.